1. Field of the Invention
This invention relates to airborne nanoparticle size classification for purposes of characterisation of the number concentration of particles in various size ranges using portable and hand-held means.
2. Description of Related Art
The motion of nanoparticles in a gas medium is influenced by diffusion. A particle undergoing diffusion travels a random, irregular path. Its position at any given time depends on the carrier gas movement and most recent collisions with molecules of the gas. Smaller particles with less momentum are more strongly affected than larger particles with greater inertia. Sub-micron and larger particles are not significantly affected by diffusion at normal temperatures and pressures. As particles undergoing diffusion pass through a fine mesh screen, some collide with the screen wires. Surface-attractive forces between particle and wire cause the particle to stick to the surface of screens. Because of diffusion, a larger fraction of small particles will collide with the screen than will large particles. Thus, the penetration of small particles will be lower than for large particles.
It is important to recognise that the nanoparticles are much smaller than the mesh opening size of the screen (normally about 20 micrometers). Therefore, the usual screen collection method for large particles, interception used in the life science and technology, is not applicable. The screen-type Diffusion Battery usually consists of ten stages. Each stage contains one or more screens. For the Model 3040 (see for instance: Model 3040/3041 Diffusion Battery Instruction Manual, TSI, St. Paul, Minn.), the first stage contains one screen, the second contains two screens, the third contains three screens, and so on. The first stage removes a large fraction of the smallest particles but allows nearly all 0.1-micrometer (□m) particles to penetrate. Each successive stage removes a larger fraction of the larger particles. Finally, by the time the aerosol has passed all ten stages, a significant fraction of the 0.1-micrometer particles have been removed.
A diffusion battery is a convenient instrument for characterising airborne nanoparticles or aerosol particles. The Model 3040/3041 Diffusion Battery classifies aerosol particles according to size in the diameter range of 2 nm to 0.2 micrometers. It has ten stages of classification and 11 sampling ports: one at the entrance and one behind each of the ten stages. Each stage consists of one or more fine mesh stainless-steel screens (the multiple screens are in series with each other). The first stage, with one screen, removes about 50 percent of the 0.006-micrometer diameter particles from the aerosol stream at a flow rate of 4 liters per minute. Aerosol that penetrates the two screens of the second stage—making a total of three screens from the entrance—has already had about 50 percent of the 0.018-micrometer particles removed. Aerosol that penetrates the ninth stage (with nine screens), making a total of 45 screens from the entrance, has had about 50 percent of the 0.20-micrometer particles removed.
The Diffusion Battery is normally used as a particle size classifier. It is typically placed upstream of an aerosol concentration detector such as a Condensation Particle Counter (CPC). The detector is first connected to the sample port upstream of the first stage of the Diffusion Battery, then downstream of the first stage, then downstream of the second stage, and so on. Since fractional aerosol penetration depends upon the size of aerosol particles, the size distribution from the particle penetration data may be evaluated. Thus, particle size distribution is determined from stage-penetration data.
The Diffusion Battery can be used with TSI's Model 3022A continuous-flow Condensation Particle Counter or Model 3025A Ultrafine Condensation Particle Counter or with any other particle detecting instrument. An automatic port selector (Model 3042 Automatic Switching Valve) is used with the Diffusion Battery and the CPC enables the flow to be directed through various stages.
A diffusion battery is a large, heavy, desktop instrument that cannot be readily employed outdoors or at a live factory. This is because a diffusion battery requires a complicated flow maintenance system (e.g. an Automatic Switching Valve) that enables the aerosol flow to be passed through different stages of a battery. In U.S. Pat. No. 4,463,595 authors suggest to use a seven-cell, parallel flow, screen-type diffusion battery for the size characterisation and size classification of ultra-fine aerosols. The parallel flow diffusion battery comprises an intake manifold for receiving an aerosol and distributing it to a plurality of diffusion cells extending through a cell holding surface. Each cell has a tubular body extending through the surface and means for diffusing an aerosol and a filter serially mounted within the tube. The density of each diffusing means differs from the density of the diffusing means for each other cell, permitting a plurality of measurements to be simultaneously taken from each aerosol sample. In a preferred embodiment, the diffusing means comprises a number of wire screens, the number of screens being different within each cell. This invention enables aerosols to be characterised without complex flow distributing system. However, aerosol filters are used in the invention to collect particles for further analysis, for instance using gravimetric technique. Therefore, it cannot be employed for on-line in situ characterisation of aerosol particles and it cannot be coupled with a single particle counter.
The particle number concentration decreases continuously as it goes through the successive diffusion elements (screens/nets) due to increase in the time particles are in the vicinity of wires of screens. In practice, concentration measurements are taken as a function of number of screens. They contain information about particle size distributions. It is important for the current invention that, in principle, the plurality of measurements can be obtained in another way (using a single screen) by means of changing the flow rate of the carrier gas containing particles. The lower flow rate the more time particles are in the vicinity of wires and the greater fraction of the particles are removed from the flow. Therefore, concentration measurements obtained at various flow rates contain similar information regarding particle size distributions as measurements with different number of screens in a conventional diffusion battery.